1. Field of the Invention
The present invention relates to apparatus for controlling the quantity of light or radiation emitted from a light emitting element to a selected level.
2. Description of the Prior Art
Light emitting elements such as a semiconductor laser are generally very sensitive to temperature and their optical output is unstable under atmospheric conditions that include variable temperature. Therefore, under the condition of variable temperature, it is required to stabilize the optical output from the semiconductor laser by employing any suitable means.
Hitherto, semiconductor lasers have often been used as a light source in optical telecommunications systems or the like and various types of circuits have already been proposed to stabilize the optical output from such semiconductor lasers. In such type of circuit the semiconductor laser is driven by an input signal obtained by modulating the carrier wave by means of digital information signals. Therefore, in this case, an effective stabilization of the optical output can be attained by feeding back the mean or maximum value of outputs. However, in another type of apparatus such as a laser beam printer, the semiconductor laser is driven directly by information signals without a carrier wave. In this case, it is impossible to use the mean-value feedback method. When the maximum value feedback method is employed, a photodetector has to be used which has a very high response speed. This is disadvantageous in view of economy.